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The Flight of Dragonfly Robots

Researchers are testing whether robotic dragonflies could be agile and elusive fliers.

One day, the U.S.
military hopes to use tiny flying robots,
equipped with cameras and sensors, for surveillance. But such robots would need
to be able to navigate around obstacles, carry weight, and be efficient enough
to fly for long periods of time. A group of researchers believe that the key to
making such a robot might lie in the dragonfly.

Beating wings: Scientists at the University of Ulm used a robotic dragonfly–about 10 centimeters tall–immersed in liquid to measure air forces around the dragonfly’s flapping wings. The green lines around the motor are lasers reflecting off of air bubbles, indicating the aerodynamic lift forces created by the wings.

Dragonflies are one of few creatures that utilize four
independently controlled wings to fly, allowing them to hover, dart, glide,
move backward, and change directions rapidly. Looking to understand such
abilities, scientists at the Royal Veterinary College,
in England, and the University of Ulm,
in Germany,
have developed a robotic dragonfly to measure the current flows over and under the
wings at different flap cycles. While most of the dragonfly hovering scenarios
were not efficient, the team found that if the lower wings are beating slightly
ahead of the top wings, the double set of wings proves more efficient at
generating lift, employing 22 percent less power to lift the same weight as a
single pair.

“The one specific advantage you get in four wings is the
maneuverability and ability to pick things out of the air and hover and dart
around,” says Jonathan How, a
professor at MIT who works on flying robots but was not involved in the
dragonfly project. “It would be really amazing if we could build something that
got anywhere near that level of performance. If you can achieve the same lift
at a lower power, that’s helpful.”

Despite their potential advantages, small flying robots that
mimic dragonflies’ agility have not been successfully made, in part because of
the complexity of the aerodynamics around four wings, and also because of fabrication issues
involved with small flying machines. However, studies of wing motion and air
forces that reveal how dragonflies achieve their agility may enable roboticists
to eventually build capable, swift flyers that use four wings.

Multimedia

To measure the air currents, the Ulm researchers immersed the robotic
dragonfly in a tank filled with mineral oil and peppered with air bubbles. Two
green lasers combined and reflected off the air bubbles as a high-speed camera took
images 10 to 20 milliseconds apart. By comparing images, the scientists
calculated the direction of flow for regions within the tank.

In terms of four-wing versus two-wing systems for a
biomimetic micro air vehicle, “it’s a trade-off,” says Fritz-Olaf
Lehmann, a researcher at the University
of Ulm who worked on the
study. With a four-wing system, the disadvantages are the need for an extra
control system and extra power. However, a system with two wings
must incorporate ways to change the angle, amplitude, and frequency of the
wings flapping to change lift, says Lehmann. Conversely, with four wings, “you
can just advance one flight system against the other, and then you change lift
production,” he says. “I think that makes building a micro air vehicle much
easier.”

In creating an autonomous micro air vehicle, “every little
bit of efficiency counts,” says Robert
Wood, a professor at Harvard
University who has
developed some of the smallest flying
robots. “You could make the argument that if you have a four-winged vehicle,
you’ll have more [control] to assist you in stabilization,” he says.

Michael
Dickinson, a professor at Caltech who works on understanding and mimicking
fly flight, says that interest in dragonflies is growing and that the Lehmann
paper is not the first containing this kind of analysis but “one in a floodgate
of papers.” While the study might add to the understanding of the subtle aerodynamics
of four-winged flight, Dickinson
points out that researchers must still develop a better, lighter battery that
powers the vehicle and makes an effective control system.

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